1. Field of the Invention
The invention relates to aqueous dispersions of polymers which, following removal of water, can be converted into elastomers or resins, to processes for preparing them, and to their use as sealants, adhesives and coating materials.
2. Background Art
In order to protect the environment, chemical formulations should desirably contain no volatile organic constituents. Accordingly, the use of aqueous systems is continually on the increase.
Aqueous dispersions of polymers, especially those which following removal of water can be converted into elastomers or resins, are by now well known. They consist essentially of a linear polymer, an emulsifier and water. Also known is the addition of further substances such as crosslinkers, adhesion promoters, crosslinking catalysts and nonreinforcing fillers. Polymers used are those containing reactive end groups. Either the finished polymers can be processed directly to an emulsion, using water and emulsifier, or starting materials for the polymers are first emulsified, optionally followed, if desired, by an emulsion polymerization, for example, of the addition polymerization, polycondensation or polyaddition type, or by radiation crosslinking or thermal crosslinking. The polymer emulsion can then be mixed with a crosslinker component and catalysts, either as they are, or in the form of an emulsion, and with further constituents, such as fillers, adhesion promoters, etc.
Previously, aqueous dispersions of organopolysiloxanes generally have been stabilized by means of organic emulsifiers. Cationic, anionic, ampholytic and nonionic emulsifiers have been used. Publications which exemplify this include EP 365 439 A and EP 665 882 A.
U.S. Pat. No. 4,221,688 and U.S. Pat. No. 4,427,811 describe silicone polymer emulsions which cure to elastomers on removal of water, which are stabilized by anionic emulsifiers. These emulsions further comprise colloidal silica in order to enhance the mechanical properties. U.S. Pat. No. 5,162,429, moreover, describes silicone polymer compositions composed of a polymer emulsion stabilized with anionic emulsifiers and an aqueous pyrogenic silica dispersion stabilized with anionic emulsifiers. The pyrogenic silica is employed therein as a reinforcing filler.
The existing aqueous dispersions of polymers which can be converted into elastomers following removal of water generally have the disadvantage of providing poor adhesion to substrates, particularly when subject to moisture. This is attributed to the high emulsifier content, which is necessary in order to produce adequate storage stability.
Around 1910, Pickering was the first to prepare paraffin-water emulsions, which he stabilized only by adding various solids, such as basic copper sulfate, basic iron sulfate or other metal salts. This type of emulsion is also referred to as a “Pickering emulsion”. Fundamental investigations have shown that one characteristic of Pickering emulsions is that particulate solids are sited at the interface between the two liquid phases, where they form a barrier which prevents the liquid droplets from coalescing.